The long-range aim of this research is to better understand voice production mechanisms which influence vocal register. Specifically, this research will focus on the neuromuscular, acoustic, biomechanic, dynamic and perceptual mechanisms which impact vocal register. While a variety of studies have investigated such mechanisms with regard to the control of fundamental frequency and vocal intensity, most of these studies were not systematic or comprehensive in nature, but were limited to a relatively small set of vocalizations which may or may not have been executed correctly. Even fewer studies have investigated such mechanisms with regard to the control of voice quality and vocal register. With an intact neuromuscular model of phonation---both with and without a vocal tract---systematic, comprehensive investigations will be performed with variations to subglottal pressure and the activation levels of specific intrinsic laryngeal muscles which have been reported to most significantly influence voice quality and vocal registers, including the cricothyroid muscle, thryroartenoid muscle, posterior cricoarytenoid muscle, and the lateral cricoarytenoid and interarytenoid muscle complex. By systematically quantifying phonatory output across a wide range of neuromuscular conditions, both with and without a vocal tract, longstanding, unresolved questions will be investigated as to whether vocal fold vibration, voice quality and vocal register may more appropriately be characterized as being vocal tract dependent or independent. More specifically, across a wide range of phonatory conditions, the level of source-tract interactions will be carefully scrutinized by comparing the vocal output generated from identical neuromuscular laryngeal configurations both with and without a vocal tract, especially when vocal instabilities occur, and when the fundamental frequency or one of its harmonics approaches the first formant of the vocal tract. A comprehensive laboratory study of this type is needed to help resolve the complex interactions between both antagonistic and synergistic muscles, which enable the laryngeal tissues to couple with the glottal airflow and/or acoustic resonances, achieve self-sustained oscillation, and produce an acoustic signal which exhibits potential changes in vocal register. In addition to the perceptual dimension of voice quality, vocal registers will also be distinguished by the resultant laryngeal vibratory mechanisms, biomechanical properties of the vocal fold tissue layers, vocal efficiency (the ratio between output sound pressure level and input power), the relative ease of phonation (the minimum lung pressure required to initiate and sustain phonation), and tissue collision dynamics which may predispose an individual to voice disorders. Thus, it is anticipated that this study will yield considerable data which will impact the prevention and treatment of voice disorders.